Consists of : 
Fetal blood
Pulmonary system
Immune system
Skin
Nervous system
Gastro-intestinal system
Urinary system
Endocrine glands
Cardiovascular
Fetal blood
Haematopoesis:
1st fetal blood cells : Day 14 embryo : in yolk sac
Next, major site is liver: 6th wk embryo : (hence, enlarged liver) & spleen (lesser extent)
Finally, bone marrow starts to produce RBC:16 wks
By 26 wks : bone marrow is the predominant source of RBC
As the fetus grows, volume of blood in fetoplacental circulation increases.
At term, 125 ml/kg fetal wt.
Hb in the fetal blood also rises.
At term, Hb 18 g/dl. Why ? Enhances the transfer of O2 across placenta.
Fetal RBC has shorter lifespan.
At term, 90 days.
Hb in fetus:
Mostly fetal haemoglobin (HbF): α2 γ2 instead of adult Hb A (α2 β2 ) and Hb A2 (α2 δ2 )
10 – 28 wks 90 % of fetal Hb is HbF
28 – 34 wks HbF: HbA= 80:20
6 mths age HbF = 1 %
HbF vs HbA
HbF has greater O2 binding capacity and it is resistant to denaturation by acid/alkali.
Fetal Haemoglobin
HbF has higher affinity for O2 than adult in vivo due to a lower sensitivity to DPG (2,3 diphosphoglyceric acid)
Partial pressure of O2 in fetal circulation is low (92.7KPa) but this is compensated for by high Hb concentration and greater oxygen affinity
CVS
Differences (fetal vs adult circulation):
Major portion of Rt ventricular output bypasses the lungs coz fetal blood does not need to enter pulmonary vasculature
Oxygenation occurs in the placenta
Right & Left ventricles work in parallel ,rather than in series
Heart, brain & upper body receive blood from left ventricle ; placenta & lower body receive blood from both right & left ventricles
Presence of shunts:
1. Ductus Venosus
2. Foramen ovale
3. ductus arteriosus
Fetal Circulation
Fetal blood pathway
Placenta ---- umbilical vein
----
ductus venosus ----
IVC ----
right atrium ----
foramen ovale ----
left atrium ----
left ventricle ----
ascending aorta- coronary/cerebral arteries SVC right atrium right ventricle pulmonary artery ductus arteriosus descending aorta umbilical artery placenta
After birth, constriction/collapse of:
umbilical arteries: instantaneous functional closure / actual obliteration takes 2-3 mths. Distal part – form lateral umbilical ligament. Proximal part – remain as superior vesical arteries
Umbilical veins: closure a little later than arteries. Forms ligamentum teres.
ductus arteriosus: functional closure soon after pulmonary circn is established. Anatomical obliteration takes 1-3 mths. Forms ligamentum arteriosum.
foramen ovale: functional closure soon after birth, anatomical closure in 1 yr.
ductus venosum: forms ligamentum venosum
Incidence of closure of fetal channels
Fetal circulatory response to hypoxia
- Heart rate falls
- Resistance in the umbilical artery increases
- Resistance in the middle cerebral artery decreases thus protecting flow to the fetal brain
- Blood flow increased to heart and adrenals
- Blood flow reduced to kidneys producing oligohydramnios (reduced volume of amniotic fluid)
Control of Fetal Heart Rate
Control of FHR is complex.
It is subject to modulating influences such as catecholamines and baroreceptors.
These influences generally act on FHR via the autonomic nervous system.
Parasympathetic tone dominates
Fetal Heart rates (FHR):
-20 weeks 155/minute
-30 weeks 144minute
-Term 140/min
FHR decreases in response to hypoxia
Fetal Lung and Breathing
Development consist of
Pseudoglandular stage: 5-16 w
Canalicular stage: 16-25 w
Terminal sac stage: type II cells begin to produce surfactant
At birth only 15% of adult alveoli are present in lungs continue to grow up to 8 years
20 wks: full differentiation of capillary & canalicular elements of fetal lung
24 wks: alveoli develop, surfactant appear.
Lung alveoli lined by surfactant (a group of phospholipids).
Prevents collapse of small alveoli during expiration by lowering surface tension
Synthesized by type 2 alveolar cells ( 10 % of lung parenchyma)
Lecithin ( phosphatidyl choline): main (80%)
Increased lecithin production :
Cortisol
Growth restriction
Prolonged rupture of membranes
Delayed lecithin formation in:
diabetes
Phosphatidyl glycerol is more predictive of RDS esp in diabetic fetus.
Surfactant:
Composition: 90% lipids, 10% proteins
Phosphatidylcholine (lecithins):
DPPC: active component(50%)
PG: second(8-15%)
At birth,with first breath, an air to tissue interface is produced in the alveolus. Surfactant spreads to line alveolus to prevent collapse during expiration
Glucocorticoids and Surfactant :
Accelerate type 2 alveolar cell development
Accelerate surfactant synthesis
Accelerate lung structural maturation
Glucocorticoids play essential role in maturation of many fetal organs but excess amounts reduce fetal growth
Fetal breathing:
Numerous but intermittent fetal breathing
movements occur in utero especially during
sleep ( lung maturation).
Immune system
8 wks: lymphocytes appear
Mid 2nd trimester: all phagocytic cells, T & B cells, complement
Fetus consists almost totally of maternal IgG (transferred across placenta).
16 wks: Maternal IgG transfer begins & increases
Last 4 wks: bulk of IgG acquired.
Hence, preterm : less IgG
Newborns produce IgG slowly & adult values are reached only after 3 yrs.
IgM & IgA: very little is produced by fetus.
not transferred across placenta.
General immunological defences:
1.Amniotic fluid (lysosomes, IgG)
2.Placenta (lymphoid cells, phagocytes, barrier)
3.Liver ( granulocytes)
Skin
16 wks: lanugo appears but disappears near term
1 month gestation till birth: skin thickness progressively increases
20 wks: stratum corneum distinct
Last wks: skin covered by vernix caseosa
desquamated skin cells
rich in cholesterol and glycogen
Preterm babies
- No vernix & thin skin ie, larger insensible loss
- Deficient brown fat
- Delayed development of sweat glands
Alimentary system & energy stores
- 10 wks: Swallowing reflex develops & gradually matures. Continuously and increasingly swallows amniotic fluid
- 2nd trimester: Peristalsis in intestines
- Term : Large bowel is filled with meconium
- But defaecation in utero (meconium in amniotic fluid) unusual unless fetal anoxia
Intestinal villi well developed by 19weeks
- Gut development important for amniotic fluid
homeostasis; (fetus swallows amniotic fluid
from 12 weeks and amniotic fluid contains
hormones and growth factors that stimulate
gut development)
- Gastrin, motilin and somatostatin regulate
growth and development present in gut by 13
weeks matures by 24 weeks
- Digestive enzymes eg disaccharidases
present by 9-10 weeks maturity at term
Meconium:
Lanugo, vernix, scalp hair, epithelial cells from skin, mucus, exfoliated intestinal epithelium, intestinal juices
Liver:
Reduced capacity for conjugation of bilirubin esp. in preterm
Preterm infants:
Virtually no fat, so severely reduced ability to withstand starvation
Incompletely developed alimentary system
Poor and unsustained sucking ability
Uncoordinated swallowing mechanism
Delayed gastric emptying
Poor absorption of carbohydrates/fat/other nutrients
Kidney & urinary tract
- Urine usu found in UB of even small fetuses
- Fetal urine gives rise to much of amniotic fluid.
(protein-free & sugar-free hypotonic ultrafiltrate of fetal plasma)
12 wks: urine production starts
32 wks: 12 ml per hour
40 wks: 38 ml per hour
- 36 wks: nephrogenesis complete
But maturation of excretory & concentrating ability of fetal kidneys is gradual
Preterm: immature, so abnormal
Amniotic fluid
Source:
- Early pregnancy: ultrafiltrate of maternal plasma
- 10 wks: transudate of fetal serum via skin & umbilical cord
- After 20 wks: mainly fetal urine ( as cornification of fetal skin, impermeable to water).
Pulmonary fluid and fluid filtering through placenta contribute little.
- AF volume:
Varies at each week of gestation.
Volume increases with increasing gestation.
10 wks: 30 ml
20 wks: 300 ml
30 wks: 600 ml
38 wks: 1000 ml
40 wks: 800 ml
42 wks: 350 ml
Functions of AF:
- Cushions fetus: Protect fetus from mechanical injury
- Permits movements of fetus while preventing limb contracture
- Prevents adhesions between fetus & amnion
- Maintains temperature
- Minimal nutritive function
- Permit fetal lung development-- 2 way movements of fluid into fetal bronchioles
- Absence of AF in 2nd trimester: pulmonary hypoplasia
- Promotes growth & differentiation of GI tract
- Renal agenesis, cystic kidney, IUGR ----oligohydramnios----reduced contribution of fluid into amniotic sac ----- major alterations in AF volume.
- Anencephaly, oesophageal atresia, duodenal atresia ----- polyhydramnios----- reduced removal of fluid
Fetal behaviour
- 18 wk: first fetal movements (quickening) perceived in primipara
Several wks earlier in multipara
- Formal counting of fetal movements (screening)
Reduced or Absent in
- Chronic hypoxia & growth failure
- fetal death
- Spinal cord extends along entire length of vertebral column in embryo, but after that, it grows more slowly.
By 24 wks: spinal cord extends to S1
At birth: L3
In adults: L1
- Myelination of spinal cord: starts mid-preg
- 10 wks: swallowing
- 14-16 wks: respiration evident
- After 24 wks: ability to suck
- 24-25 wks: can hear some sounds in utero
- 28 wks: sensitive to light
Fetal endocrinology
- 10 wks: GH, ACTH, PRL , TSH produced by fetal pituitary
- 1st trimester: Vasopressin, oxytocin from posterior pituitary
- Fetal adrenal: hypertrophy of reticular zone (site of synthesis of DHEA-dehydroepiandrosterone). This zone is absent in anencephaly. Adrenal medulla produces catecholamines.
- 11 wks: Fetal thyroid produces small amount thyroxine
- Fetal ovaries remain inactive
- Fetal testicles mediate development of male reproductive structures
- 12 wks: fetal pancreas secrete insulin
0 comments:
Post a Comment